The outlined proposal is directed to a better understanding of the processes of determination, differentiation and pattern formation. These processes are common to all developing systems and therefore most likely have similar underlying mechanisms controlling them. We plan to study these problems during oogenesis, embryogenesis and metamorphosis in Drosophila, three crucial stages during development. By (1.1) interfering with specific inhibitors during oogenesis, (1.2) by altering normal interaction of egg regions during early development with ligation and transplantation of cytoplasm and by investigating mosaic animals consisting of normal and pattern affecting mutations we anticipate finding information on the origin and position of factors responsible for the determination of body parts in the embryo. The experiments on imaginal discs are designed to elucidate cell interactions which alter the normal fate of cells (e.g. regeneration). Clonal analysis will determine the origin and capabilities of such cells and in combination with pattern mutations to what extent developmental decisions are irreversible. Furthermore it is of importance to know if all cells in a disc at a particular stage in development are in the same physiological state and are able to respond to the hormonal signals during metamorphosis, an aspect little investigated in pattern formation studies.